Process for decomposing intermetallic compounds in metals

ABSTRACT

A PROCESS FOR AT LEAST PARTIALLY DECOMPOSING INTERMETALLIC COMPOUNDS IN METAL AND METALLOID-CONTAINING ALLOYS. THE INVENTION IS CHARACTERIZED BY HEATING THE ALLOY TO A TEMPEATURE AT LEAST AS HIGH AS THE MELTING TEMPERATURE OF THE INTERMETALLIC COMPOUND BUT BELOW THE MELTING TEMPERATURE OF AT LEAST ONE OF THE METALS OR METALLOIDS IN THE ALLOY FOR A PERIOD OF TIME SUFFICIENTLY LONG TO THERMALLY DISSOCIATE AT LEAST A PORTION OF THE INTERMETALLIC COMPOUND AND THEN RAPIDLY COOLING THE ALLOY TO MINIMIZE RE-FORMATION OF THE DISSOCIATED COMPOUND BY BLOWING THE MELT.

United States Patent 3,704,117 PROCESS FOR DECOMPOSING INTERMETALLIC COMPOUNDS IN METALS Alex R. Valdo, Baton Rouge, La., and Freeman M.

Sanderford, Dalton, Ga., assignors to Ethyl Corporation, New York, N.Y.

No Drawing. Continuation-impart of appplication Ser. No. 744,307, July 12, 1968. This application July 21, 1971, Ser. No. 164,952

Int. Cl. B22d 23/08 U.S. Cl. 75.5 C 8 Claims ABSTRACT OF THE DISCLOSURE A process for at least partially decomposing intermetallic compounds in metal and metalloid-contaim'ng alloys. The invention is characterized by heating the alloy to a temperature at least as high as the melting temperature of the intermetallic compound but below the melting temperature of at least one of the metals or metalloids in the alloy for a period of time sufficiently long to thermally dissociate at least a portion of the intermetallic compound and then rapidly cooling the alloy to minimize re-formation of the dissociated compound by blowing the melt.

CROSS-REFERENCES TO RELATED APPLICATIONS This application is a continuation-in-part of U.S. application Ser. No. 744,307, filed July 2, 1968, now Patent No. 3,615,343.

This invention is directed toward a novel technique for purification of alloys, and aluminum alloys in particular. Since many alloys contain undesirable impurities, it is frequently necessary to remove these impurities or at least reduce the quantity thereof before beneficial use can be made of the alloy. Consequently, there presently exists a need for a simple and inexpensive method for purifying alloys by reducing the quantity of undesirable impurities, such as intermetallic compounds, in these alloys.

Accordingly, it is an object of this invention to provide a process for reducing the content of undesirable intermetallic compounds in various alloys by thermally causing the compounds to dissociate.

Another object of the invention is the purification of alloys which can be utilized commercially after removal of certain impurities.

In accordance with this invention, there is provided a process for reducing the content of one or more undesirable intermetallic compounds in an alloy by heating the alloy to a high enough temperature for a sufiiciently long time to dissociate at least a portion of the intermetallic compound and then rapidly cooling the alloy by blowing the melt to minimize re-formation of the dissociated intermetallic compound. This heating causes a decomposition of the intermetallic compound or compounds to the constituent elements and the subsequent quenching retards re-formation of these undesirable compounds by effecting a rapid temperature drop through the formation temperature of the compound.

The invention is characterized by convenient flexibility in that substantially any alloy containing one or more undesirable intermetallic compounds can be purified provided the alloy is capable of undergoing peritectic decomposition. The term peritectic may be defined as an isothermal reversible reaction in which a molten liquid phase reacts with a solid phase to produce another solid phase on cooling. This invention is directed toward the elimination of, or at least reduction in, the amount of 3,704,117 Patented Nov. 28, 1972 certain intermetallic compounds present in this solid phase formed when the molten alloy is cooled. Thus, the term peritectic decomposition is used to denote the process of reducing the quantity of certain intermetallic compounds originally present in the alloy. This is accomplished by first heating the alloy to a temperature above the formation temperature of the intermetallics to dissociate these compounds and then rapidly quenching the alloy by blowing the melt to minimize the re-formation of the undesirable intermetallic compounds in the solid phase produced by cooling the molten alloy. In this manner it has been found that a solid phase may be formed which contains innocuous amounts of the compound sought to be eliminated.

Substantially any alloy may be easily purified according to the techniques of this invention, magnesium alloys, zinc alloys and the like being exemplary. Alloys containing aluminum are particularly well suited to upgrading by the instant process. Aluminum alloys which are readily susceptible to purification may have an aluminum content varying from a fraction of a percent up to 99 percent and higher.

Aluminum alloys which are particularly susceptible to purification by this process are those containing from about 40% to about aluminum by weight. The aluminum may be alloyed with substantially any element so long as one or more intermetallic compounds are present, the removal of which is desired.

rExemplary of the metals and metalloids which may be present as intermetallics with aluminum in these alloys are iron, carbon, silver, boron, calcium, cobalt, chromium, manganese, nickel, titanium, uranium, vanadium, zinc, copper and silicon. Two or more of these, and other elements may be present in varying proportions with aluminum and may easily be removed from the alloy by the process of this invention. For example, common aluminum alloy-intermetallic constituents which readily undergo peritectic decomposition and which are therefore susceptible of being removed by application of the inventive process are FeAl MnAl CuMgAl TiAl3, FeSi AL; and Al2Tl5Sl Of the foregoing intermetallic compounds which are frequently found to be undesirable constituents of aluminum alloys, compounds containing at least 2 of the elements, aluminum, silicon, titanium, iron, copper, magnesium and manganese are particularly significant because they are frequently found in useful alloys such as those obtained by the carbothermic reduction of clay.

Alloys manufactured by the carbothermic reduction technique quite frequently contain, in addition to free aluminum, varying quantities of aluminum, silicon and iron as well as aluminum and titanium, in intermetallic form. Common intermetallic compounds containing these metal combinations are FeSi Al and TiAl although it will be recognized that many other combinations are also frequently found. It has been found that by application of this invention such undesirable intermetallic compounds may be thermally decomposed, thereby enriching the aluminum content of the base aluminum alloy.

As previously noted, a basic feature of the invention is the heating of an impure alloy to raise its temperature at least to, and preferably above, the melting point of the intermetallic compound which is to be decomposed in the alloy. Suitable temperatures which may be utilized in the invention are dependent upon the melting temperature of the intermetallic compound which is to be reduced in any given alloy. In general, however, temperatures in the range of from about 300 C. to about 1400 C. are high enough to melt intermetallic compounds which are susceptible to peritectic decomposition. For example, if decomposition of the intermetallic FeSi Al is to be accomplished in an aluminum alloy containing by weight about 68% aluminum, 27% silicon, 3% iron and 2% titanium, the alloy should be heated to a temperature of at least 870 C. (which is the melting temperature of this intermetallic compound) before rapid cooling is effected. Similarly, if it is desirable to remove TiAl from an alloy having the same or a similar composition, a temperature of at least 1340 C. should be attained before quenching the alloy.

In order to insure that substantially all of the intermetallic compound is melted, it is preferable to heat the alloy to a temperature somewhat above the melting point of the intermetallic, sufficiently above the melting point to yield a fluid melt. In blowing from the melt, the alloy containing the intermetallics must be heated sufficiently high to melt the metal. The metal must be in a fluid state before it can be blown.

The length of time at which the alloy should remain at the intermetallic melting point is not critical, but it should be long enough to insure that all of the intermetallic compound or compounds to be decomposed are reduced to the molten state. Generally, a time interval of at least 1 minute is suflicient for alloys heated to temperatures within the range of from about 300 C. to about 1400" C. More preferably, the alloy should be held at the appropriate temperature from 2 to about 60 minutes in order to achieve the proper degree of intermetallic dissociation time when the molten alloy has been subjected to stirring.

After the alloy from which undesirable intermetallic compounds are to be removed is heated to the appropriate temperature, it should be rapidly cooled in order to prevent the reformation of these compounds, as heretofore noted. Rapid cooling is accomplished by blowing the melt, the procedure for which is described hereinafter.

The equipment satisfactory for carrying out the blowing of a molten alloy consists of a graphite crucible equipped with an induction heating coil wrapped around the outside of the crucible so that the charge can be heated to the desired temperature by induction. The crucible has a hole in the bottom thereof to which is attached a graphite tube. A graphite rod is placed in the crucible to close the graphite tube and contain the molten alloy in the crucible until the blowing operation is conducted. Twentyfour air jets are mounted concentrically around the graphite tube in the shape of a cone with the apex of the cone pointed downward so that the stream of air which is discharged from the jets is directed to the discharge end of the graphite tube. This assembly is mounted on a refractory slab which in turn is mounted on a metal cylinder. The graphite tube and air jets extend through a hole in the refractory cover so that the blown powder is retained by and collected in the metal cylinder. The metal cylinder is equipped with openings at its top to serve as vents for the air discharged through the air jets.

The alloy to be treated is charged as lumps to the graphite crucible. Alternatively, the alloy can be prepared in situ by charging to the crucible the proper amounts of the different pure metals required to produce an alloy of the desired composition. The crucible and charge are then heated by induction to the desired temperature to yield a homogeneous, fluid mel of alloy. The molten alloy is maintained at the desired temperature for minutes or for sufficient time to assure that complete melting has occurred and a homogeneous melt is obtained. The in-- duction field induces a gentle rolling action within the melt which enhances homogenization.

After the melt is maintained at the selected temperature for an adequate period, a valve is opened to admit air to the jets mounted around the bottom of the graphite tube. The graphite rod is then removed from the crucible, and the molten alloy flows through the graphite tube and is atomized by the air from the jets. The atomized alloy falls downward in the metal cylinder, cools and solidifies and is collected in the bottom of the cylinder. If the metal cylinder is tall enough, the alloy will be cooled sufliciently by the flow of air. If it is not practical to use a tall cylinder, a pool of. water can be placed in the bottom of the metal cylinder to effect final cooling and solidification.

The particle size of the blown powder can be controlled by varying the air pressure applied to the air jets and/or the diameter of the tube through which the molten alloy is discharged from the crucible. In general, the higher the air pressure, the smaller the particle size of the product alloy; the smaller the tube diameter, the smaller the particle size. However, the tube diameter cannot be so small that the molten alloy cools and solidifies in the tube. In general, the finer the particle size the more rapid is the cooling of the molten alloy and the less is the concentration of undesirable intermetallics in the final product.

This apparatus has been used by others to prepare pure metal powders and alloys. For example, nickel powder has been prepared by personnel in the Canadian Department of Mines using an apparatus and procedure similar to the foregoing.

Variations of the above apparatus and procedure can be employed :with equal effectiveness. Hence, instead of having the air jets impinging on the stream of molten metal to atomize the molten metal, the air can be introduced into the tube above its discharge end and the alloy forced through a nozzle attached to the end of the tube to atomize the molten metal. Gases other than air such as argon, nitrogen may be used provided they do not react with an contaminate and the alloy to any great extent. A stream of water may be used in place of air, and the use of argon instead of air is especially suitable when it is desired to produce an alloy powder having a minimum oxygen content. Materials of construction may be used other than graphite, the main requirement being that they are compatible with the molten alloy.

Generally, a smaller residue of the intermetallic compound sought to be removed has been found in the alloy subjected to melt blowing as compared to an alloy of the same composition which has been subjected to other quenching techniques. It is preferable to quench the alloy at a rate of from about to about 400 degrees per second, and most preferably, about 200 degrees per second.

Accordingly, a preferred embodiment of the invention is characterized by heating an alloy containing aluminum, silicon, iron and titanium, wherein the intermetallic compounds to be decomposed are composed of aluminum, silicon and iron and aluminum and titanium, respectively, to a temperature within the range of from about 300 C. to about 1400 C. and then cooling the molten alloy by rapidly blowing the melt. In a more preferred embodiment, the alloy contains by weight about 68% aluminum, 27% silicon, 3% iron and 2% titanium, the intermetallic compounds to be decomposed are FeSi Al and TiAl the alloy is heated in an inert atmosphere to a temperature of about 1340 C. and held at this tempertaure for about 15 minutes, or until the melt is sufliciently fluid after which the alloy is cooled by rapidly blowing the melt.

As heretofore mentioned, it is desirable to decompose certain unde'sirbale intermetallic compounds present in alloys without simultaneously producing additional undesirable compounds. Consequently, the inventive process defines a technique for at least partially decomposing intermetallic compounds in a variety of metal and metalloid-containing alloys by first heating an alloy to a temperature at least as high as the melting temperature of the intermetallic compound to be decomposed but below the melting temperature of at least one of the metals or metalloids in the alloy, the yield of which it is desired to increase. This temperature should be maintained for a time sufiiciently long to thermally dissociate at least a portion of the intermetallic compound,and then the alloy should be rapidly cooled by blowing the melt.

This technique of controlled heating promotes the dissociation of undesirable intermetallics in the alloy to elemental, innocuous components. Subsequent rapid cooling of the alloy by blowing the melt substantially preserves these components by retarding re-formation of the undesirable intermetallics and minimizing the formation of other undesirable compounds containing the metal or metalloid, the presence of which is desired in elemental form in the alloy. In this aspect of the invention a preferred alloy for upgrading is composed of aluminum, silicon, iron and titanium and the intermetallic compounds to be decomposed contain at least 2 of the elements, aluminum, silicon, titanium, iron, copper, magnesium and manganese.

In a preferred embodiment of this aspect of the invention where the alloy contains the metalloid, silicon, it is desirable to heat the alloy to a temperature above the melting point of the intermetallic compound or compounds to be decomposed but below the melting temperature of the silicon, maintain this temperature for a time sufliciently long to dissociate the intermetallic constituents and then rapidly cool the alloy in order to increase the yield of silicon in the cooled alloy. As above noted, this technique retards re-formation of the original intermetallic compounds in the alloy and also substantially reduces the amount of silicon-containing compound in the quenched alloy. In this manner, the quantity of elemental silicon or other element in a given alloy may be increased. As in previous embodiments of the invention, the temperature may generally be within the range of from about 300 C. to about 1400 C. and should be maintained for at least one minute or until the melt is fluid. Further, the cooling stage of the process is effected by rapidly blowing the melt.

In a most preferred embodiment of the controlled heating aspect of the invention the alloyto be purified contains by weight about 68% aluminum, 27% silicon, 3%

.iron and 2% titanium and the intermetallic compounds to be decomposed are FeSi Al Al Ti Si and TiAl Further, the alloy is heated to a temperature of about 1340 C. (which is below the melting point of silicon) in an inert atmosphere such as nitrogen or argon, where it is held for about 15 minutes or until the melt is fluid. The cooling phase is effected by rapidly blowing the melt.

This invention and the various embodiments thereof may be further understood by the following illustrative examples.

Example I A sample of Al-Si-Fe-Ti alloy, having the composition by weight 62% Al, 33% Si. 2% Fe, 3% Ti, was melted at l250 C. to give a homogeneous liquid which was then atomized by allowing it to flow through a A" tube and directing a stream of water at 200 psi. onto the molten metal stream. The resultant powder Was 95% 30 mesh and 85% 100+ mesh. X-ray diffraction analysis of the resultant powder gave a diffraction pattern which contained no peaks assignable to the FeSi Al intermetallic and only two small peaks which might be assignable to an Al-Si-Ti intermetallic. Hence, the product contained at most trace amounts of the Al-Si-Ti intermetallic. On the other hand, a comparison sample having essentially the same composition but which was cooled and crystallized slowly gave an X-ray pattern which showed the presence of small, but detectable amounts of FeSi Al and about 6% 0f the Al-Si-Ti intermetallic.

Example II An aluminum-silicon-iron-titanium alloy composition was prepared by placing weighed amounts of the individual metals in a graphite crucible and heating the cru cible in an induction coil to 1400 C. to form a fluid homogeneous melt which was held at 1400 C. for 15 minutes. The molten alloy was then forced through a discharge tube under argon pressure and atomized by blowing the metal through a nozzle connected to the end of the discharge tube. The blown alloy was analyzed by X-ray fluorescence (XRF), X-ray diffraction (XRD) analysis of the powder showed it to contain little or no intermetallics as set forth in Table I hereinafter.

Two other Al-Si-Fe-Ti alloy compositions were prepared by melting weighed amounts of the individual metals in a graphite crucible. However, these samples were not blown from the melt. Instead, the crucible and molten alloy were removed from the furnace and allowed to cool to room temperature. The solid alloy samples were removed from the crucibles, crushed and ground. X-ray fluorescence (XRF) and X-ray diffraction (XRD) analyses of these samples are also set forth in Table I.

TABLE I.ALLOY COMPOSITIONS Percent element in Percent intermetallics in alloy (XRD) alloy (XRF) Sample number Al Si Fe 'Ii FeSizAh AlzTisSiB FezTi 1 None. 3 1. 4 Trace.

It can be clearly seen that both cast and ground samples (2A and 2B) contained more intermetallics than did the sample (1) blown from the melt.

Example III An alloy composition was prepared and blown according to the procedure of Example II. The composition of the resultant powder was determined by X-ray fluorescence to be 56.0% A1, 32.7% Si, 3.1% Fe and 2.8 Ti. X- ray diffraction analysis detected 2% FeSi A1 and 5% Al Ti Si no Fe Ti was detected.

For comparison, a second alloy sample was prepared, but was cooled, crushed and ground (instead of being blown). Its composition was determined by X-ray fluorescence to be 55.2% A1, 28.5% Si, 3.4% Fe and 2.8%, Ti. It was found to contain by X-ray diffraction 3% FeSi Al 7% Al Ti Si and about 1% Fe Ti.

Again, it has been clearly demonstrated that substantially less intermetallics are found when the alloy has been rapidly cooled by blowing the melt.

From an analysis of the preceding examples and embodiments it is apparent that the invention provides an easy and economical method for decomposing a quantity of undesirable intermetallic compound in virtually any alloy capable of undergoing peritectic decomposition. Operating procedures are simple and necessary equipment is inexpensive and is readily available. Accordingly, the invention contributes to the art of alloy purification.

The foregoing disclosure and description of the invention is illustrative and explanatory thereof and various changes may be made within the scope of the appended claims without departing from the spirit of the invention.

What is claimed is:

1. A process for at least partially decomposing intermetallic compounds in metal and metalloid-containing alloys which comprises, in combination, the steps of heating the alloy to a temperature at least as high as the melting temperature of the intermetallic compound but below the melting temperature of at least one of the metals or metalloids in the alloy for a period of time sufficiently long to thermally dissociate at least a portion of the intermetallic compound and then rapidly cooling the alloy by blowing the melt to minimize re-formation of the intermetallic compound.

2. The process of claim 1 wherein said alloy is an aluminum alloy.

3. The process of claim 1 wherein said intermetallic compound contains at least two of the elements, aluminum, iron, carbon, silver, boron, calcium, cobalt, chromium, manganese, nickel, titanium, uranium, vanadium, zinc, copper and silicon.

4. The process of claim 1 wherein said intermetallic compound is a first compound composed of aluminum, silicon and iron, a second compound composed of aluminum and titanium and a third compound composed of aluminum, titanium and silicon.

5. The process of claim 1 wherein:

(a) said alloy contains aluminum, silicon, iron and titanium, and

(b) said intermetallic compounds contain at least two of the elements, aluminum, silicon, titanium, iron, copper, magnesium and manganese.

6. The process of claim 5, wherein said at least one of the metals is silicon.

7. The process of claim 1 wherein said alloy is heated to a temperature within ther ange of from about 300 C. to about 1400 C. and held at this temperature for at least 1 minute.

8. The process of claim 1 wherein:

(a) said alloy contains by weight about 68 percent aluminum, 27 percent silicon, 3 percent iron and 2 percent titanium,

melt.

References Cited UNITED STATES PATENTS 2,967,351 1/1961 Roberts et al 75-0.5 C 3,150,975 9/1964 Beaver et a1. 75-226 15 WAYLAND STALLARD, Primary Examiner US. Cl. X.R. 264-12, 13 

